1. Field of the Invention
This invention relates to rotary head devices, and more particularly to rotary head devices having a plurality of heads rendered operative in sequence to record or reproduce signals.
2. Description of the Related Art
Recently, in the field of video cassette recorders (VCRs), to realize minimization of the size and diameter of the rotary head assembly, a type of recorder has become known wherein a magnetic tape is wound around the rotary head assembly over a wider angle range than 270.degree. and four rotary heads which rotate in phase intervals of 90.degree. are used in sequence when signals are recorded or reproduced.
When an erasing head is with a rotary head cylinder having such four recording/reproducing heads, the accompanying rotary transformer requires five channels.
In general, such a multiple-channel rotary transformer has a shortcoming in the that the adjacent two of the channels are cross-talked. The method for lessening this is to separate the core itself, or to create an intervening groove a short-circuit ring (coil) occupies, or to arrange the channels so that the adjacent two of them are not used simultaneously, and the coils of the channels not in use are short-circuited.
Of these, the 5-channel rotary, transformer using the short-circuit ring is exemplified in FIG. 1 where seven coils 1A to 1G are arranged in respective annular grooves 2A to 2G formed in a core 3 of the rotor, and another seven coils 1A' to 1G' are arranged in respective annular grooves 2A' to 2G' formed in a core 3' of the stator. Of these coils 1A-1G and 1A'-1G', the 1C, 1F, 1C' and 1F' are the short-circuit rings. So, there is little cross-talk between the coil 1B-1B' and the coil 1D-1D' and between the coil 1E-1E' and the coil 1G-1G'
FIG. 2 shows a head arrangement in the rotary head cylinder using such a transformer. 4 is a rotary head drum. On its outer periphery, a tape 5 is wound obliquely over a range of more than about 270.degree. by posts 6A and 6B. Four heads 7A to 7D are arranged on the rotary drum 4 with their phases differing from each other by 90.degree., and are switched over successively in the order: 7A.fwdarw.7B.fwdarw.7C.fwdarw.7D.fwdarw.7A and so on, when signals are recorded or reproduced. Also, to prevent the signals from dropping out when each head 7A-7D is switched over, the angle of winding of the tape 5 is made equal to 270.degree.+2.theta..sub.0.degree.. The switching over from the head 7D to the head 7A is carried out when both heads 7D and 7A lie just in the positions shown in FIG. 2. An erasing rotary head 8 is set up in a delayed position from the head 7A in a clockwise direction by an angle of .theta..degree.. The combinations of connection between each head 7A-7D and 8 and each coil 1A-1G of the rotary transformer are so chosen that the adjacent coils 1A and 1B, and coils 1C and 1D are not used simultaneously. Such combinations are eight in number as shown in the table of FIG. 3.
FIG. 4 shows a timing chart for switching over the heads and the transformer when in the recording or reproduction mode with the employment of the combination II. At first, when recording, while erasing current is allowed to always flow to the coil 1G which is connected to the erasing head 8, the flow of recording current to the coils 1A, 1E, 1B and 1D which are connected to the recording/reproducing heads 7A to 7D respectively is switched over in sequence at a timing depending on the phase of rotation of the heads 7A to 7D as shown in the drawing. In the waveforms of FIG. 4, the period of H level is the period during which the recording current flows. In the period of L level, the coils of the stator are short-circuited to produce a similar effect to that of the short-circuit rings 1C and 1F. By taking into account the fact that when the heads are switched over, its switching-over timing is deviated from the coincidence with the supply timing of the recording current, the recording current must be made to flow simultaneously to the adjacent two heads, for example, the heads 7A and 7D for a period shown by t.sub.O in the figure. Because the corresponding coils 1A and 1D to these two heads 7A and 7D are not adjacent to each other, each other's interference due to the cross talk does not take place. Also, the coils which are connected to any adjacent two of the other heads are not adjacent to each other, or lie on the opposite side of the short-circuit ring 1C. Therefore, the problem of the cross-talk between the recording currents does not arise.
Further, because the short-circuit ring 1F intervenes between the coil 1G for the rotary erasing head 8 and the other coils for the heads, there is no possibility of occurrence of the erasing current leaking into the recording current.
Even when in the reproduction mode, the problem of cross-talk does not arise from the similar reason.
However, the above-described rotary transformer has seven grooves and seven coils. To improve its characteristics, therefore, the size is necessarily increased, giving rise to a problem in that it is not suited to a minimization of the size of the rotary head device.
For the characteristics of the transformer, for example, the transmission loss, can be more improved as the confronting area for each channel increases, and the larger the number of turns of the coil, the easier the good characteristics are obtained.